A testosterone test measures the level of this male hormone (androgen) in the blood. Testosterone affects sexual features and development. In men, it is produced in large amounts by the testicles. In both men and women, testosterone is also produced in small amounts by the adrenal glands; and, in women, by the ovaries. A test to measure testosterone can be done to: Determine whether a problem with the testicles or pituitary gland is preventing a man from being able to father a child (infertility). A low amount of testosterone can lead to low sperm counts. Evaluate a man's sexual problems. A low level of testosterone may reduce a man's sex drive or cause impotence (erectile dysfunction). Determine whether increased testosterone production is causing a boy younger than age 10 to have early (precocious) puberty. Evaluate why a woman is developing male features, such as excessive facial and body hair (hirsutism) and a deep voice. Evaluate irregular menstrual periods in women. Determine the response to testosterone-lowering medications in men with advanced prostate cancer. Evaluate the cause of osteoporosis in a man.